Josh,
for my reply im going to assume you aren't aware of the ins-and-outs of camshaft timing. say, the average joe with a family who loves his explorer and just wants a bit more grunt out of it. a guy with better things to do then sit at the computer into the wee hours of the morning reading about cams. please understand that by no means am i trying to call you stupid. yes, cam timing changes many variables (and i suppose it can change DYNAMIC compression), but i wouldnt say it's your simplest power adder. im going to try to make this as simple as i can, but it's still huge. if you want Comp Cams' opinion, or dont understand mine, go
here.
just so you know, my cam knowledge comes from magazines, Desktop Dyno, a passion for a thump to my '69 chevy's 350, and a shop teacher who has built motors for 30 years. (im a high school student, big deal.)
cam timing crash course (from a guy who learns about cams into the wee hours of the morning):
im going to assume that we know how a basic OHV 4-stroke engine works. from now on, im also going to use the terms TDC, BDC, IVO, IVC, EVO, and EVC - any cam junkie is familiar with these as they make or break a cam. the first two are "top dead center", and "bottom dead center", which refer to where the piston is in it's stroke.
the next 4 stand for (and represent the actions of) Intake Valve Opening, Intake Valve Closing, Exhaust Valve Opening, and Exhaust Valve Closing. these figures are always represented in degrees of either [before or after] TDC/BDC. this might sound like mumbo-jumbo, but bear with me.
okay, so big deal, the motor turns and we get a sequence:
combustion, EVO, IVO, EVC, IVC, next combustion.
"wait, what? are you saying that the intake opens before the exhaust closes?" you might ask. yup! look at this comp cam graph:
those curves look a lot like cam lobes, dont they? coincidence? hell no.
the valve may only be cracked open 0.050" at TDC, but the valve is still open and air is still flowing. why? because on a naturally aspirated engine, air&fuel is introduced through vaccum, right? the open exhaust port creates that continuing suction through the chamber as the piston rises. when both valves are open, this is called VALVE OVERLAP.
your cam is designed to cause the valves to behave in a specific manner, corresponding to the different times of the crankshaft's rotation. makes sense, right? im basically saying the valves move in harmony with the crank.
that said, by changing the cam timing, you're moving the whole cam graph by whatever amount of degrees. for arguement purposes, and to make it easier on me, let's say you're moving by 10 degrees ADVANCED.
so, at the TDC in the middle of the cam profile at officially 0 degrees, instead of only a little bit of each valve being open, your new TDC behaves as it would at the old -10 degrees. why?
because by changing your timing, you're telling everything to happen 10 degrees before it was designed to. an appropriate analogy is a carefully designed Olympic runner who starts precisely early at the 100m sprint.
still with me? good! now that we have our newly timed valves (or olympic runners, if it helps) starting early, let's look at what this changes.
look at 0 degrees, TDC. our intake valve is closed, but the exhaust valve is open a bit more. this means that we're kicking more used gasses out, but injecting less fuel into the combustion chamber at the moment. thats fine, but how does that affect flow?
unfourtunately for you i cant tell you what formula tells you because i have no idea. just take my word for it that flow will change. and when flow changes, what else does? horsepower, and compression!
I have read that advancing the cam can lead to an increase in compression. Is this true?
that depends on what compression you're looking for.
if you're looking for a ratio like 9.0:1, or
STATIC COMPRESSION, your answer is
no. but if you're looking for a PSI value at the spark plug, or
DYNAMIC COMPRESSION, the answer is yes. why?
look at the cam profile. when the valves are overlapped, there is no compression occuring. why? because any air that tries to compress gets kicked out of the exhaust valve and stays at the same pressure.
what happens if we re-time the cam to the point where when the piston moves up the stroke towards TDC, no valves are open and the air MUST compress? well then pressure would increase because more air molecules are being forced together and none have anywhere else to go.
"so my answer is yes, right? alright, cool! ill go whip out the tools!" is what i hope you arent saying. there's a few more things to consider.
chances are very high that by blindly advancing a cam you can gain some dynamic compression and your engine will pickup a few ponies. this is a pretty good roll of the dice because to be fair, the factory designs a cam with other things besides power in mind.
idle quality is a big factor, only a select few like their SUVs lumpy at a stoplight. soccer moms would rather have their trucks purr at a stoplight because it vibrates less and makes less "weird noises", yes? so your cam is designed to purr at idle like a happy kitty.
but that's besides the point. whole new set of conditions: assume your cam has been re-timed and the exhaust valve is fully open when the piston is at TDC. is your OHV motor what's called an
"INTERFERENCE" engine? -insert blank confused stare- that means that if your timing belt/chain were to snap and come out of
harmonius timing, your valves can physically collide with the piston. yeah. so that means if you dick with the timing to the point where the valve can touch the piston, and at 250rpm when you hit the key, there's a lot of force involved. unfourtunately, valves bend before pistons stop moving, so then you have a bent valve which can lead to a whole multitude of problems i hope i dont have to explain.
i could go into the process of degreeing the cam, in an OHV engine, it involves tearing the intake, valve covers, pushrods, lifters, and rockers out. in short? it pretty much is a total engine overhaul!
finally, lets assume that the compression increases and all goes well.
how much does it increase by? can you still run pump gas? will it detonate?
what happens if it detonates and you fry a piston? uh-oh... total engine overhaul...
so, to conclude this amass of jargon that i've spelled out?
-cam timing can increase dynamic compression.
-cam timing is a pain in the ass to do.
-cam timing depends on many factors, including cam spec, head volume, and whether your engine is an interference-type or a non-interference-type.
-cam timing is a pain in the ass to do.
to be honest josh, i would look at other things to do before re-degreeing an OHV motor. do you have a cold air intake on your truck? hows the exhaust? how's the plugs? what is your rear axle ratio? changed the computer yet?
i've probably skipped something elementary, but hopefully this somehow helps.
ian
Very good write up that I'm sure will help others as well. I really appreciate you taking the time to write this up. As far as the interferance engine part, I actually saw that on extreme 4x4 yesterday. As for a lumpy idle, I would love that. Nothing sounds better than a big lumpy cam. I always pride myself by thinking outside the box. 
